1. Field of the Invention
The present invention relates to computer system input devices such as a digitizers referred to as touchpads and more particularly to the systems and methods necessary to acquire signals from such input devices, to condition the acquired signals to eliminate noise, and to convert the acquired and conditioned signals to digital codes that are transmitted to a computer system.
2. Description of Related Art
Touchpads are small digitizer based devices that allow a person to write or draw upon the surface of the touchpad and have the signals and codes from a controller to be interpreted by a computer system. The touch pad digitizers may be of three types, capacitive, resistive and electromagnetic.
Referring to FIG. 1, the surface 12 of the touchpad becomes a "writing surface" for capturing the position of an pointed object 10 such as a finger, pen or stylus upon the touchpad. The touchpad signals are analog signals that will be captured by a touchpad interface circuit 28 and translated to digital codes that will be transferred to a computer system 32 on an interface 30. The interface 30 may be an industry standard serial interface, an industry standard parallel interface, or a custom interface requiring special adapter circuitry within the computer system 32 to accept the digital codes from the touchpad interface 28.
An example of a resistive touchpad is shown in FIG. 1. The resistive touchpad is made up of multiple layers of resistive films and protective layers. The protective hard coating 12 is the surface onto which the pointed object 10 is pressed upon during the writing and drawing. A first layer of resistive film 14 is attached to the protective hard coating 12 on the surface opposite the writing surface. This first layer of resistive film forms the Y-plane of the touchpad. Attached to the surface of the Y-plane resistive film 14 opposite the surface attached to the hard protective coating 12 is a second resistive film 16. This second resistive film 16 forms the X-plane of the touchpad. Finally attached to the side of the X-plane resistive film 16 is a supporting back layer 18. This back layer provides protection and mechanical support for the for the X-plane and Y-plane resistive films 14 and 16.
The touchpad interface 28 is connected through the touchpad interface lines 20, 22, 24, and 26. Each line will provide a stimulus such as a current or voltage to the periphery of the X-plane resistive film 16 and the Y-plane resistive film 14. As shown in FIG. 2, as the pointed object 10 is pressed 40 on the touchpad surface 12, the Y-plane resistive film 12 will deform and touch the X-plane resistive film 14. The X-plane resistive film can not deform because it is supported by the supporting back layer 18. This causes the Y-plane resistive film 14 and the X-plane resistive film 16 to come into contact with each other. This will cause a response in the form a change in voltage or current depending upon whether the stimulus from the touchpad interface 28 of FIG. 1 is a constant voltage or a constant current. If the stimulus from the touchpad interface 28 of FIG. 1 is a constant voltage the currents through the touchpad interface lines 20, 22, 24, and 26 will be modified according the position of the pointed object 10 on the touchpad surface 12. However, if the stimulus from the touchpad interface 28 of FIG. 1 is a constant current the voltages between the touchpad interface lines 20, 22, 24, and 26 will be modified according to the position of the pointed object 10 on the touchpad surface 12.
Referring back to FIG. 1, the touchpad interface 28 will have a set of analog to digital converters that will sense the change in the analog responses from the touchpad interface lines 20, 22, 24, and 26 and convert them to digital codes indicating the absolute position of the pointed object 10 upon the touchpad surface 12. The digital codes may be transmitted directly to the computer system across the interface 30 and translated to absolute coordinates within the computer system or the touchpad interface 28 may determine the absolute coordinates and transmit them directly to the computer system 32. For the computer system 32 to use the absolute coordinates generated by the touchpad interface 28 to control the movement of the cursor 36 upon the display screen 34, these absolute coordinates must be modified to codes that define the relative motion of the cursor 36. The relative motion will be the speed and direction of the cursor 36 as it is moved across the display screen 34. The modification from absolute coordinates to relative motion information must be done within an internal mouse emulation program resident within the computer system 32.
As the pointed object 10 is placed upon the touchpad surface 12 and is moved in the path 11, the hand 5 will cause the motion of the path 11 to vary as a result of vibration or rocking of the hand 5 or the hand 5 will vary in pressure causing the center location 40 of the indentation shown in FIG. 2 to vary. This will cause unwanted changes in the digital codes being sent to the computer system 32. Additionally, the hand 5 will have electrical noise that will be coupled to the analog signals on the touchpad interface lines 20, 22, 24, and 26.
If there is dirt or moisture on the surface 12 of the touchpad, the analog signals will be corrupted causing unwanted variations in the digital codes being transferred to the computer system 32. Other noise that may effect the analog signals on the touchpad interface lines 20, 22, 24, and 26 may be internal to the computer system 32 such as from the computer system clocking subsystem, drivers on the nearby integrated circuit chips that will control the display 34, and other electromagnetic noise present within the computer system 32.
The unwanted variations in the digital codes may cause unwanted results such as the cursor 36 moving erratically across the display screen 34 or strokes or sweeps of that use the absolute locations of the pointed object 10 upon the touchpad surface 12 will become misinterpreted causing errors in handwriting recognition or graphics programs.
U.S. Pat. No. 5,327,161 (Logan, et al.) describes a method to emulate mouse input devices using a program resident within a computer system. A touchpad input device has a controller that generates a digital code that contains the absolute position of a pen or finger on the mouse pad. This requires a special interface that is unique to the touchpad circuitry. Additionally, this patent describes a method for the continuation of cursor movement when a pointed object is touching the touchpad and has been moved on the touchpad to a special border area. The pointed object must be stopped within the border for the continuous motion to be engaged. The direction of the scrolling may be made as a modification of the original direction and velocity of the pen prior to the transiting and stopping within the border area of the touchpad. This modification will be made as a change in the velocity of the movement of the cursor along an axis parallel to the edge of the touchpad adjacent to the border area where the pointed object is resting.
U.S. Pat. No. 5,376,946 (Mikan) describes a circuit using an EPROM to convert signals from a touch screen adhered to a computer display screen to digital codes of the industry standard computer input mouse protocols.
U.S. Pat. No. 5,543,590 (Gillespie, et al.) describes a capacitive sensor system that can detect the location of a finger or stylus on a sensor matrix. The location is determined and translated as electrical signals for use in other circuitry such as a computer system to control a cursor upon a display screen.
U.S. Pat. No. 5,543,591 (Gillespie, et al.) discloses methods for recognizing tapping, pushing, hopping and zigzagging gestures upon a conductive sensor pad that can be interpreted into cursor control motions such as clicking, double clicking, and click and drag use with computer mouse devices. Further this patent also describes the use of a running average for X and Y position data.
U.S. Pat. No. 5,488,204 (Mead) shows a proximity sensor system that has a capacitive touchpad. The capacitance of the touchpad changes with the proximity of an object to the touchpad. As the capacitance changes a voltage changes which is converted to electrical signals representing the X and Y coordinates of the object on the touchpad. A conductive paint brush stylus is used to produce paint-like strokes on a display screen associated with the touchpad. The system also incorporates features that allow the stylus to emulate the actions of a computer mouse including an "edge motion" where a cursor can be moved over long distances without rowing or stroking the stylus.
U.S. Pat. No. 5,287,119 (Drumm) describes a computer input device using orientation sensors. Orientation sensors are mounted on the head of an operator. The movement of the head of the operator will cause a cursor on a computer display screen to move. Part of the algorithm for the control of the data from the sensors involves averaging of a present value of orientation with a previous value of orientation before transmission of the orientation data to the computer system.
U.S. Pat. No. 4,193,118 (Nash et al) describes a circuit to extract a low frequency signal component from a composite signal composed of a high frequency signal and a low frequency signal. An analog-to-digital converter will convert the composite signal to a digital output signal. A storage register carries a running average of the composite that represents the low frequency component of the composite signal.